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Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| Constraining Disk Parameters of Be Stars using Narrowband Hα Interferometry with the Navy Prototype Optical Interferometer
Interferometric observations of two well-known Be stars, γ Cas andφ Per, were collected and analyzed to determine the spatialcharacteristics of their circumstellar regions. The observations wereobtained using the Navy Prototype Optical Interferometer equipped withcustom-made narrowband filters. The filters isolate the Hαemission line from the nearby continuum radiation, which results in anincreased contrast between the interferometric signature due to theHα-emitting circumstellar region and the central star. Because thenarrowband filters do not significantly attenuate the continuumradiation at wavelengths 50 nm or more away from the line, theinterferometric signal in the Hα channel is calibrated withrespect to the continuum channels. The observations used in this studyrepresent the highest spatial resolution measurements of theHα-emitting regions of Be stars obtained to date. Theseobservations allow us to demonstrate for the first time that theintensity distribution in the circumstellar region of a Be star cannotbe represented by uniform disk or ringlike structures, whereas aGaussian intensity distribution appears to be fully consistent with ourobservations.
| Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type starsmembers of the subgroups Upper Scorpius, Upper Centaurus Lupus and LowerCentaurus Crux of the nearby Sco-Cen OB association. The radial velocitymeasurements were obtained by means of high-resolution echelle spectravia analysis of individual lines. The internal accuracy obtained in themeasurements is estimated to be typically 2-3 km s-1, butdepends on the projected rotational velocity of the target. Radialvelocity measurements taken for 2-3 epochs for the targets HD 120307, HD142990 and HD 139365 are variable and confirm that they arespectroscopic binaries, as previously identified in the literature.Spectral lines from two stellar components are resolved in the observedspectra of target stars HD 133242, HD 133955 and HD 143018, identifyingthem as spectroscopic binaries.
| A search for fine structure inside high resolution profiles of weak diffuse interstellar bands
This paper presents a survey of the high-resolution profiles ofselected, moderately weak diffuse interstellar bands (DIBs) between 4725and 6730 Å. In very high signal-to-noise spectra, obtained as aresult of averaging several individual exposures of reddened, early-typestars that show Doppler splitting of <2 km s-1 ininterstellar gas lines, the profiles seem to have a substructure. Thissupports the molecular origin hypothesis for DIBs. We studied theprofiles of the diffuse interstellar bands at wavelengths of 4726.33,4963.85, 5418.89, 5541.74, 5544.95, 5546.46, 5762.73, 5766.05, 5769.09,6439.41, 6445.53, 6449.16, 6729.28 Å.
| Astrophysical laser operating in the OI 8446-Åline in the Weigelt blobs of η Carinae
Within the framework of a simple model of photophysical processes in theWeigelt blobs in the vicinity of the luminous blue variable (LBV) starη Carinae, we explain the presence of the fluorescent 8446-Åand forbidden [OI] 6300-Ålines as well as the absenceof the allowed OI 7774-Åline in spectra recorded with the HubbleSpace Telescope (HST)/STIS instrument (Gull et al.). From atomic dataand estimated stellar parameters we demonstrate that there is apopulation inversion and stimulated emission in the3p3P-3s3S transition λ8446 due tophotoexcitation by accidental resonance (PAR) by H Lyβ radiation.
| The initial conditions of isolated star formation - VI. SCUBA mappingof pre-stellar cores
Observations have been carried out with the Submillimetre Common-UserBolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT) ofregions of comparatively isolated star formation in molecular cloudcores. Some 52 starless cores were observed, which are molecular cloudcores that do not contain any sign of protostellar activity such asinfrared sources or bipolar outflows. These are all therefore candidatepre-stellar cores, which are believed to represent the stage of starformation that precedes the formation of a protostar. Of the 52 cores,29 were detected at 850 μm at varying levels of signal-to-noise ratiogreater than 3σ at peak, while 23 of the cores were observed butnot detected. The mean detection lower limit of the data correspondsroughly to an AV~ 15 under typical assumptions. The detectedcores were split into `bright' cores and `intermediate' cores, dependingon their peak flux density at 850 μm. Those with peak 850-μm fluxdensities greater than 170 mJy beam-1 were designated`bright' cores. Those with peak 850-μm flux densities less than thisvalue were designated `intermediate' cores. This dividing linecorresponds to a mean detection limit of 10σ at peak, and anapproximate AV~ 50 under typical assumptions. Of the 29detected cores, 13 are found to be bright and 16 are intermediate. Thedata are combined with our previously published ISO data, and thephysical parameters of the cores, such as density and temperature, arecalculated. The bright cores are detected with sufficiently highsignal-to-noise ratio to allow their structure to be mapped. Radial fluxdensity profiles of these show flattened inner regions and sharpboundaries, consistent with previous observations of pre-stellar cores.Detailed fitting of the bright core radial profiles shows that they arenot critical Bonnor-Ebert spheres, in agreement with previous findings.However, we find that intermediate cores, such as B68 (which haspreviously been claimed to be a Bonnor-Ebert sphere), may in fact beconsistent with the Bonnor-Ebert criterion, suggesting perhaps thatcores pass through such a phase during their evolution. We also findthat the masses of the bright cores have a mean value of approximatelythe same order as their virial masses. We make rough estimates of corelifetimes based on the statistics of detections and find that thelifetime of a pre-stellar core is roughly ~3 × 105 yr,while that of a bright core is ~1.5 × 105 yr.Comparisons with some models that regulate collapse using eithermagnetic fields or turbulence show that no model can match all of thedata. Models that are tuned to fit the total pre-stellar core lifetimedo not predict the relative numbers of cores seen at each stage.
| Dust Properties in the Far-Ultraviolet in Ophiuchus
We have derived the albedo (a) and phase function asymmetry factor (g)of interstellar dust grains at 1100 Å using archival Voyagerobservations of diffuse radiation in Ophiuchus. We have found that thegrains are highly forward-scattering, with g=0.55+/-0.25 anda=0.40+/-0.10. Even though most of the gas in this direction is in theOphiuchus molecular cloud, the diffuse FUV radiation is almost entirelydue to scattering in a relatively thin foreground cloud. This suggeststhat one cannot assume that the UV background is directly correlatedwith the total amount of gas in any direction.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| Be Star Hα Line Profile Variability
The monitoring of the spectroscopic variability of Be stars is crucialfor testing Be star models. Motivated by this, a Be star monitoringproject was developed for undergraduate student research involvement. Wehave been obtaining 0.8 Angstrom/pixel resolution Hα line profilesfor several bright Be stars since 2003 June. These spectra were acquiredusing the UCA Fiber Fed Spectrograph used at the UCA Observatory and theNubbin Ridge Observatory in Royal, AR. H-α line profiles,velocities, and variability are shown for Delta Sco, Chi Oph, Eta PsA,48 Lib, and Upsilon Sgr (HD181615).Funding has been provided by the UCA University Research Council and theArkansas Space Grant Consortium.
| On the relation between diffuse bands and column densities of H2, CH and CO molecules
Mutual relations between column densities of H2, CH and COmolecules as well as between the latter and strengths of the major 5780and 5797 diffuse bands are presented and discussed. The CH radical seemsto be a good H2 tracer, possibly better than CO. It is alsodemonstrated that the molecular fraction of the H2 moleculeis correlated with an intensity ratio of 5797 and 5780 DIBs, suggestingthe possible formation of narrow DIB carriers in denser clouds,dominated by molecular hydrogen and reasonably shielded from ionizing UVradiation by small dust grains.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/949
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Observations of Rotationally Resolved C3 in Translucent Sight Lines
The rotationally resolved spectrum of theA1Πu<--X1Σ+g000-000 transition of C3, centered at 4051.6 Å, hasbeen observed along 10 translucent lines of sight. To interpret thesespectra, a new method for the determination of column densities andanalysis of excitation profiles involving the simulation and fitting ofobserved spectra has been developed. The populations of lower rotationallevels (J<=14) in C3 are best fitted by thermaldistributions that are consistent with the kinetic temperaturesdetermined from the excitation profile of C2. Just as in thecase of C2, higher rotational levels (J>14) ofC3 show increased nonthermal population distributions inclouds that have been determined to have total gas densities below ~500cm-3.
| Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and13CO was detected toward ρ Oph A and χ Oph. Themeasurements were obtained at medium resolution with the Goddard HighResolution Spectrograph on the Hubble Space Telescope. Column densityratios, N(12CO)/N(13CO), of 125+/-23 and 117+/-35were derived for the sight lines toward ρ Oph A and χ Oph,respectively. A value of 1100+/-600 for the ratioN(12C16O)/N(12C18O) towardρ Oph A was also obtained. Absorption from vibrationally excitedH2 (v''=3) was clearly seen toward this star aswell. The ratios are larger than the isotopic ratios for carbon andoxygen appropriate for ambient interstellar material. Since for bothcarbon and oxygen the more abundant isotopomer is enhanced, selectiveisotopic photodissociation plays the key role in the fractionationprocess for these directions. The enhancement arises because the moreabundant isotopomer has lines that are more optically thick, resultingin more self-shielding from dissociating radiation. A simple argumentinvolving the amount of self-shielding [from N(12CO)] and thestrength of the ultraviolet radiation field permeating the gas (from theamount of vibrationally excited H2) shows that selectiveisotopic photodissociation controls the fractionation seen in these twosight lines, as well as the sight line to ζ Oph.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| An Ultra-high-Resolution Survey of the Interstellar 7Li/6Li Isotope Ratio in the Solar Neighborhood
In an effort to probe the extent of variations in the interstellar7Li/6Li ratio seen previously,ultra-high-resolution (R~360,000), high signal-to-noise spectra of starsin the Perseus OB2 and Scorpius OB2 associations were obtained. Thesemeasurements confirm our earlier findings of an interstellar7Li/6Li ratio of about 2 toward ο Per, thevalue predicted from models of Galactic cosmic-ray spallation reactions.Observations of other nearby stars yield limits consistent with theisotopic ratio of ~12 seen in carbonaceous chondrite meteorites. If thisratio originally represented the gas toward ο Per, then todecrease the original isotope ratio to its current value an order ofmagnitude increase in the Li abundance is expected, but it is not seen.The elemental K/Li ratio is not unusual, although Li and K are formedvia different nucleosynthetic pathways. Several proposals to account forthe low 7Li/6Li ratio were considered, but noneseems satisfactory. Analysis of the Li and K abundances from our surveyhighlighted two sight lines where depletion effects are prevalent. Thereis evidence for enhanced depletion toward X Per, since both abundancesare lower by a factor of 4 when compared to other sight lines. Moreover,a smaller Li/H abundance is observed toward 20 Aql, but the K/Habundance is normal, suggesting enhanced Li depletion (relative to K) inthis direction. Our results suggest that the7Li/6Li ratio has not changed significantly duringthe last 4.5 billion years and that a ratio of ~12 represents most gasin the solar neighborhood. In addition, there appears to be a constantstellar contribution of 7Li, indicating that one or twoprocesses dominate its production in the Galaxy.
| Non-radially pulsating Be stars
Based on more than 3000 high-resolution echelle spectra of 27 early-typeBe stars, taken over six years, it is shown that the short-term periodicline profile variability of these objects is due to non-radialpulsation. The appearance of the line profile variability depends mostlyon the projected rotational velocity v sin i and thus, since all Bestars rotate rapidly, on the inclination i. The observed variability ofthe investigated stars is described, and for some of them line profilevariability periods are given for the first time. For two of theinvestigated stars the line profile variability was successfully modeledas non-radial pulsation with l=m=+2 already in previous works. Since Bestars with similarly low v sin i share the same variability properties,these are in general explainable under the same model assumptions. Theline profile variability of stars with higher v sin i is different fromthe one observed in low v sin i stars, but can be reproduced by thesame model, if only the model inclination is modified to more equatorialvalues. Only for a few stars with periodic line profile variability thel=m=2 non-radial pulsation mode is not able to provide a satisfyingexplanation. These objects might pulsate in different modes (e.g.tesseral ones, l != |m|). Almost all stars in the sample show traces ofoutburst-like variability, pointing to an ephemeral nature of themass-loss phenomenon responsible for the formation of the circumstellardisk of early-type Be stars, rather than a steady star-to-disk masstransfer. In addition to the variability due to non-radial pulsationpresent in most stars, several objects were found to show other periodsresiding in the immediate circumstellar environment. The presence ofthese secondary periods is enhanced in the outburst phases. Short-livedaperiodic phenomena were clearly seen in two stars. But, given theunfavourable sampling of our database to follow rapid variability oftransient nature, they might be more common. Only in two out of 27 starsshort-term spectroscopic variability was not detected at all.Based on observations collected at the European Southern Observatory atLa Silla, Chile, 55.D-0502, 56.D-0381, 58.D-0697, 62.H-0319, 63.H-0080,64.H-0548, and 267.D-5702, the German-Spanish Astronomical Centre, CalarAlto, operated by the Max-Planck-Institute for Astronomy, Heidelberg,jointly with the Spanish National Commission for Astronomy, and onobservations with the Wendelstein 80-cm and the Ondřejov 2-mtelescopes, both equipped with the HEROS spectrograph provided by theLandessternwarte Heidelberg.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Estimation of the mass loss, opening angle and mass of Be circumstellar disks from Brmathsf γ continuum emission and interferometric measurements
Using the SIMECA code developed by Stee & Araùjo(\cite{stee1}); Stee et al. (\cite{stee2}) for Be stars we obtain acorrelation between the mass loss rates {dot M} and the Brgammacontinuum luminosity as a function of the opening angle of the disk. Weshow that this correlation is similar to those obtained by Scuderi etal. (\cite{scuderi}) for O-B supergiants. We found that the wind densityat the base of the photosphere, from a sample of 8 Be stars, liesbetween 10-13 and 10-12 g cm-3. We alsopresent a relationship between the mass of the circumstellar disk andthe 2.16 mu m flux. Finally we emphasize how interferometricmeasurements can help to estimate the wind density and we present asample of 16 Be stars with predicted visibilities that can be observedwith the VLTI.
| Properties of STAR Star Circumstellar Envelopes Deduced from fe II Emission Lines
Not Available
| Autocorrelation Analysis of Hipparcos Photometry of Short-Period Be Stars
We have used Hipparcos epoch photometry and a form of autocorrelationanalysis to investigate the amplitude and timescale of the short-periodvariability of 82 Be stars, including 46 Be stars that were analyzed byHubert & Floquet using Fourier and CLEAN analysis and 36 other Bestars that were suspected of short-period variability. Our method hasgiven useful information for about 84% of these stars; for the rest, thetime distribution of the Hipparcos epoch photometry limits thecapability of our technique.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Fine structure of profiles of weak diffuse interstellar bands
We present a very high resolution (R=120 000-220 000) analysis ofprofiles of selected diffuse interstellar bands (DIBs): 4964, 4980,4985, 5850 Å. The 4980 Å feature is a very weak one, yetsome substructures are proved to be present, as well as within theprofiles of another stronger ones where this phenomenon was foundseveral years ago. The presence of substructures inside DIB profilessupports their molecular origin. Based on data collected at the ESO 3.6m telescope operated on La Silla Observatory, Chile and 2-m telescope ofthe Terskol Observatory, Russia.
| The interstellar C3 chain molecule in different interstellar environments
We present an analysis of spectra of six stars taken with highresolution (R=220 000). The stars are reddened by molecular clouds thatdiffer by the relative strength of the 5797 and 5780 diffuseinterstellar bands (DIBs). The high signal-to-noise ratio of the spectra(S/N ~ 700-1000) shows that the abundance of the linear moleculeC3 with respect to EB-V varies considerably fromone star to an other. There is no correlation with EB-V. Thestrong variations in the abundance of C3 must therefore becaused by another circumstance. We point out that this may be the case:from an analysis of the interstellar potassium lines in the same spectrawe conclude large differences in the state of ionization produced byinterstellar photons with energies below the ionization potential ofhydrogen. The ratio of the abundances of C3 and C2varies considerably in different directions, even when the ratio betweenthe strengths of various DIBs remains approximately constant. Based ondata collected at the ESO 3.6 m telescope operated on La SillaObservatory, Chile.
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| Takýmyýldýz: | Yilanci |
| Sað Açýklýk: | 16h27m01.40s |
| Yükselim: | -18°27'23.0" |
| Görünürdeki Parlaklýk: | 4.42 |
| Uzaklýk: | 149.925 parsek |
| özdevim Sað Açýklýk: | 0 |
| özdevim Yükselim: | 0 |
| B-T magnitude: | 4.507 |
| V-T magnitude: | 4.281 |
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